The cross-section or diameter of textile products, such as yarns and the like must be monitored at all stages of production. Because of the large number of measuring points, this demands measuring devices having the greatest possible simplicity and economic value, but nevertheless offering reliability and durability.
Optical measuring devices which have proven successful are, inter alia, those in which the yarn is irradiated from at least one side by a light source and, on the opposite side, a light receiver converts the size of the shadow produced by the yarn (i.e. the residual light incident outside the shade) into an electrical signal.
An essential prerequisite for a usable cross-section determination is that the measuring field of light intercepted by the yarn is as homogeneous as possible, consists of light of uniform intensity at all points of the measuring field, because it cannot be avoided that the position of the yarn varies within the measuring field during successive measurements. It is therefore not permissible for the measuring field to have different values within its region of influence as this would falsify the measured values.
Different measures for homogenizing the measuring field of optical measuring devices have already been disclosed, but these did not achieve the required objective. Thus, for example, it has been proposed as disclosed in German Offenlegungsschrift 2,310,204, to place in each case at least one optical fiber between the light source or light receiver and the yarn, the ends of the fiber(s) which are facing the yarn being opposite one another in axial alignment. However, this did not make it possible to achieve an adequate homogeneity.